The present invention relates generally to metering piston pumps and more particularly to piston-type fuel pumps for feeding a metered fuel supply to a combustion chamber.
In known metering pumps of the type to which the present invention relates for small amounts of liquids of the type contemplated herein, particularly where a delivery rate of two liters per hour or less is involved, complete filling of the outlet chamber of the pump is of significant importance if metering accuracy is to be maintained. Complete filling of the pump cylinder is achieved in known, single-action metering pumps utilized for pumping low-boiling point liquids, e.g. fuel injection pumps, by means of a second pump which feeds the flowing fluid under excess pressure to the inlet line of a metering stage of the device. These pumps, which are mostly provided with inlet control by the piston, feature relatively high metering accuracy. However, they tend to be generally quite complicated and to have a rather large overall volume. At least three flow-influenced valves are required, and these may include one suction valve and one pressure valve each on the feed pump and at least a pressure valve on the metering stage of the mechanism.
In order to reduce the production costs and the space requirements of such pumps, a preceding delivery or feeding stage may be eliminated with the metering stage performing the function of the suction phase of operation.
In such known proportioning pumps, the inlet of fluid is controlled by a flow-controlled suction valve. However, it has been found that a volume increase caused by the suction valve introduces disadvantageous operating conditions as does the resistance to flow of the suction valve.
In other known single-action metering piston pumps, particularly pumps for delivering fuel to a furnace, the fluid inlet is controlled by the piston. In such devices, undesired volume increases may be maintained relatively small but it is found that vacuum effects and cavitation appear in the pump cylinder. Particularly when the pump is operating to deliver low boiling point liquids, undesired steam bubbles are formed which prevent complete filling of the pump cylinder and thus detract from a high metering accuracy.
The present invention is aimed at providing a compact metering pump of the aforementioned type which features higher metering accuracy with lowered production costs and with lowered space requirements while at the same time permitting delivery of low boiling point liquids.